For electronics, microelectronics and microelectromechanics, semiconductor wafers with extreme requirements made of global and local flatness, single-side-referenced local flatness (nanotopology), roughness and cleanness are required as starting materials (substrates). Semiconductor wafers are wafers composed of semiconductor materials, more particularly compound semiconductors such as gallium arsenide and predominantly elemental semiconductors such as silicon and occasionally germanium.
In methods of the art, semiconductor wafers are produced in a multiplicity of successive process steps, wherein, in the first step, by way of example, a single crystal (rod) composed of semiconductor material is pulled by means of the Czochralski method or a polycrystalline block composed of semiconductor material is cast, and, in a further step, the resulting circular-cylindrical or block-shaped workpiece composed of semiconductor material (“ingot”) is separated into individual semiconductor wafers by means of wire saws.
In this case, a distinction is made between single-cut wire saws and multiple wire saws, designated hereinafter as MW saws (MW=multiple wire). MW saws are used, in particular, when a workpiece, for example a rod composed of semiconductor material, is intended to be sawn into a multiplicity of wafers in one work step.
U.S. Pat. No. 5,771,876 describes the functional principle of a wire saw suitable for producing semiconductor wafers. The essential components of these wire saws include a machine frame, a feed device and a sawing tool consisting of a web (“wire web”) composed of parallel wire sections.
An MW saw is disclosed in EP 990 498 A1, for example. In this case, a long sawing wire coated with bonded abrasive grain runs spirally over wire spools and forms one or more wire webs.
In general, the wire web is formed from a multiplicity of parallel wire sections which are clamped between at least two wire guide rolls, wherein the wire guide rolls are mounted in rotatable fashion and at least one of them is driven. The wire guide rolls are usually provided with a coating, for example polyurethane. In accordance with DE 10 2007 019 566 B4 and DE 10 2010 005 718 A1, a multiplicity of grooves for guiding the sawing wire are cut into the coating of the wire guide roll, wherein all the grooves have a curved groove base and groove flanks with a specific opening angle.
The longitudinal axes of the wire guide rolls are generally oriented perpendicularly to the sawing wire in the wire web.
The production of wafers composed of semiconductor material makes particularly stringent requirements of the precision of the slicing process. For this purpose, it is important that the multiplicity of grooves on the wire guide roll in which the sawing wire is guided run exactly parallel and the grooves and the sawing wire lie in one line (alignment).
In accordance with DE 102 37 247 A1, the distance between two grooves on the wire guide roll can be identical or decrease from the wire inlet side to the wire outlet side. The reduction of the distance between the grooves takes account of the circumstance that the diameter of the wire decreases during the sawing process, such that as a result of the reduced distance between grooves, despite a thinner sawing wire, wafers of uniform thickness are sliced from the workpiece.
As a result of the continuous contact with the sawing wire, the coating of the wire guide rolls and the grooves cut into the coating are subject to wear and have to be regularly renewed in order to avoid so-called alignment errors. Alignment errors, for example if the groove of the wire guide roll and the wire lying in said groove no longer lie on a straight line, can lead to damage, for example saw marks or grooves, on the surfaces of the sliced semiconductor wafers.
If the grooves of the wire guide roll are worn, they have to be renewed. For this purpose, generally the wire guide roll has to be demounted from the wire saw and it is necessary to introduce a new coating and/or new grooves into the surface of the coating of the wire guide roll (grooving of the wire guide roll). This work that has to be carried out regularly leads to outage times of the wire saw and thus restricts the economic viability of the method.
It is therefore desirable to lengthen the period of time between the renewal of worn grooves, i.e. grooving the wire guide roll. This can be done for example by coating the wire guide roll with a suitable material.
Wire guide rolls having a coating composed of polyurethane (PU) are usually used. Polyurethanes are largely resistant to the liquid abrasive medium (slurry) containing loose solids (abrasives) or to abrasion on account of sawing wires having bonded grain.
However, the coating of the wire guide roll must be neither too soft nor too hard. In the case of a material that is too soft, the coating is not sufficiently resistant to the plastic deformation of the grooves, thus resulting in an undesirable alteration of the groove geometry. In the case of a material that is too hard, the frictional engagement between wire and wire guide roll can no longer be ensured, as a result of which the wire web can no longer be optimally set in motion.
In order to reduce the wear of the coating of a wire guide roll, JP 2006102917 A2 proposes a coating in the form of urethane comprising 5 to 30 percent by weight of silicon carbide grain, such that the coating is significantly harder than just a coating consisting of urethane.
Although a harder coating lengthens the period of time between the renewal of worn grooves, it has the disadvantages mentioned above.